1. Field of the Invention
The present invention relates to an anti-skid apparatus for use in an automotive vehicle, and more particularly, to an anti-skid apparatus for installation in a vehicle braking system between a master cylinder and wheel brake cylinders to prevent the vehicle wheels or road wheels from locking in braking operation.
2. Description of the Prior Art
As one of such anti-skid apparatus as described above. Japanese Patent Application laid open in 1983 under publication No. 58-199258 discloses an anti-skid apparatus which comprises: (a) a regulator valve arranged to regulate a hydraulic pressure supplied by a fluid pump and produce a hydraulic power pressure in accordance with a hydraulic braking pressure from a master cylinder; (b) a piston arranged to be moved forward by a hydraulic braking pressure of a hydraulic circuit between a wheel brake cylinder and the master cylinder, whereby the volume of this hydraulic circuit is increased, and moved backward by the hydraulic power pressure, whereby the volume of the hydraulic circuit is decreased; (c) a normally open cut-off valve disposed in a hydraulic circuit between the piston and the master cylinder and arranged to close this circuit during the forward and backward movement of the piston; (d) a changeover valve arranged to apply the hydraulic power pressure to the piston or discharge it therefrom dependent upon a locking condition of the road wheels; and (e) a bypass valve arranged to bypass the normally open cut-off valve and connect the master cylinder to the wheel brake cylinder when the hydraulic power pressure produced by the regulator valve becomes lower than the pressure to be regulated in accordance with the hydraulic braking pressure from the master cylinder.
In the anti-skid apparatus described above, when the changeover valve is inoperative, usual braking operation is achieved, and when the changeover valve is actuated under such a condition that the hydraulic power pressure is higher than the pressure to be regulated in accordance with the hydraulic braking pressure, the anti-locking or anti-skid operation of the road wheels is achieved. When the changeover valve is actuated under such a condition that the hydraulic power pressure is lower than the pressure to be regulated in accordance with the hydraulic braking pressure, the bypass valve acts to maintain the usual braking operation, whereby a defect resulted from such movement of the piston as being held in its forward position without moving backward is prevented to be caused.
In the above-described anti-skid apparatus, however, the normally open cut-off valve and the bypass valve are provided with individually, so that the apparatus tends to be expensive, large and heavy. In addition, the apparatus disclosed in the above publication, the hydraulic power pressure is utilized only for moving the piston backward, a brake booster such as a vacuum booster is needed for a boost operation. Accordingly, the braking system as a whole tends to be expensive, large and heavy.